There are several different techniques for producing optical fiber for use in communications. One such technique comprises directing a constantly moving stream of reactants and oxygen through a glass substrate tube having a generally circular cross-section. The oxygen stream carries silicon tetrachloride and dopants to produce the desired index of refraction in the finished optical fiber. The substrate glass is heated to a reaction temperature within a moving hot zone that traverses the length of the tube, and the consequent reaction produces doped silicon dioxide fused into a continuous layer on the inner wall of the tube. The resulting tube is referred to as a preform tube. See for example, U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980 in the names of J. B. MacChesney and P. B. O'Connor.
A torch assembly for heating a glass substrate tube to facilitate deposition of the reactants in the above-described process is described in U.S. Pat. No. 4,231,777 which issued on Nov. 4, 1980, in the names of B. Lynch and F. P. Partus. See also U.S. Pat. No. 4,401,267 which issued on Aug. 30, 1983 in the name of C. D. Spainhour. Initially, one end of the tube is supported in the headstock of a lathe and the other end is welded to an exhaust tube that is supported in the tailstock. Combustible gases are directed through a housing and gas outlets of the torch assembly and toward the tube as it is turned rotatably about its longitudinal axis and as the torch assembly is moved therealong on a carriage to produce a moving hot zone. A temperature profile is produced across the hot zone which moves along on the surface of the tube to accomplish the desired reaction and deposition. See F. P. Partus, and M. A. Saifi "Lightguide Preform Manufacture" beginning at page 39 of the Winter 1980 issue of the Western Electric Engineer.
During a deposition mode, the torch carriage moves slowly from the headstock of the lathe where dopants are moved into the glass tube to the tailstock where gases are exhausted. At the end of each pass from headstock to tailstock, the torch carriage is returned rapidly to the headstock for the beginning of another cycle. The ends of the gas outlets adjacent to the tube are cooled to eliminate substantially degradation by oxidation or reduction, for example, of the material forming the housing and gas outlets. In one embodiment of this technique, a plasma is established in the tube to enhance certain processes in reaction and deposition.
Subsequent to the deposition mode, a collapse mode is used to cause the preform tube to become a solid rod-like member which is called a preform. It is this preform from which lightguide fiber is drawn. See D. H. Smithgall and D. L. Myers "Drawing Lightguide Fiber" beginning at page 49 of the hereinbefore identified Winter 1980 issue of the Western Electric Engineer.
In order to collapse the preform tube, the torch assembly is moved in a number of passes from the headstock to the tailstock and then in a plurality of passes from the tailstock to headstock. The temperature of the moving hot zone which is higher during the collapse mode than during the deposition mode softens the tube wall and allows surface tension to cause the tube to collapse into a rod. During the collapse mode, straightening methods disclosed in U.S. Pat. No. 4,477,273 which issued on Oct. 16, 1984 in the names of B. Lynch and F. P. Partus may be used to cause the resultant preform to be substantially straight. The process of collapsing a preform tube may consume as much time as four and one-half hours.
There has long been a desire to reduce the time required to collapse a preform tube into a preform. A solution to this problem will yield significant dividends as the costs are directly proportional to the time required for this step.
Seemingly, the prior art is devoid of a solution to this problem. Techniques have been proposed but none has met wide acceptance. An acceptable solution to this problem which should be able to be used with present straightening techniques should yield a lower cost preform having exceptional straightness.